Prepress color proofing systems are in widespread use in the printing industry. In a typical process, a multicolor original is separated into individual transparencies, called color separations, for the three primary colors and for black. The separation process can be carried out in a number of well-known ways; for example, a graphic arts scanner can be used to create the color separations. In some instances, more than four color separations are employed. A color proof, called an off-press proof or a prepress proof, is then prepared from the color separations. The color proof is used by the printer to check color balance and other important quality control parameters. Generally speaking, off-press color proofs are made by one of three methods; namely (1) an overlay process which employs an image on a separate base for each color, (2) a single sheet process is which the separate color images are combined on a single base, and (3) a digital method in which the images are produced directly on a single base from digital data. The overlay process and single sheet process are discussed in some detail in U.S. Pat. No. 4,895,787 which points out that the single sheet process is greatly preferred because the superposed supports of the overlay process drastically alter the appearance of the color proof.
U.S. Pat. No. 3,622,320, issued Nov. 23, 1971, describes a color proofing process of the single sheet type that is highly advantageous in that it is simple and convenient to use and particularly in that it is a dry process which does not require the use of processing solutions. As described in the '320 patent, the color-proofing process utilizes two or more light-sensitive donor elements each of which contains a different dispersed colorant. The donor elements employ a light-sensitive resin which has a tackifying point that is raised by light exposure, i.e., a resin that is photohardenable. The imagewise-exposed donor element is pressed into contact with a receiver while heated to a temperature greater than the tackifying point before exposure but less than the tackifying point in the exposed, areas, whereby the colored resin transfers from the unexposed areas to the receiver. Second, third and subsequent donor elements, each containing a different colorant, are also exposed and the image therefrom is transferred, in exact registration, to the same receiver. For full color reproduction, it is customary to expose and transfer images from donor elements containing, respectively, yellow, magenta, cyan and black colorants. Commonly, the donor elements are exposed by use of a set of half-tone color separation positives.
A full color proof is typically made by transfer of the black image, followed by the magenta image, then the cyan image and finally the yellow image; although the order of transfer is not critical and a different order can also be used depending on operator preference. Between each transfer step, the transferred image can be hardened by exposure to a suitable light source in order to prevent back transfer, i.e., transfer to a subsequently applied donor element.
In lieu of using a photohardenable resin, acceptable results in a single sheet process can also be achieved by using a resin which is phototackifiable. With this type of resin, the exposed areas will be more tacky at the transfer temperature than the unexposed areas and, accordingly, the colored resin will transfer from the exposed areas to the receiver. In this instance, the donor elements can be exposed by use of a set of half-tone color separation negatives. Phototackifiable compositions and their use in a single sheet process are described, for example, in U.S. Pat. No. 5,108,868.
Whether photohardenable or phototackifiable resins are used, in either instance the process utilizes heat to increase the difference in surface adhesion between exposed and unexposed areas and pressure to bring about effective image transfer.
To achieve the imagewise transfer of the colored resin in the process of the '320 patent, the exposed donor element is placed in contact with the receiver, the sandwich that is so formed is passed between a pair of heated pressure rollers, and the donor element is separated from the receiver which then carries the transferred image. Separation of the donor element from the receiver is usually accomplished by a manual peeling step, but can, if desired, be accomplished automatically through the functioning of the laminator. After the first image is transferred, it is critical that there be exact registration of each of the subsequently transferred images.
A very significant disadvantage of the dry color proofing system of the '320 patent is the difficulty that is encountered in accurately registering the images that are successively applied to the receiver. A common way of doing this is by means of a pin registration system, but this is inconvenient to use and often insufficiently accurate. Moreover, if an accurate pin registration system is devised it will add significantly to the cost of the color proofing process and still will not easily permit the preparation of "scatter proofs", namely proofs using one large sheet for the creation of multiple proofs of small size.
The present invention is directed to the objective of providing an improved donor element for use in a dry color proofing process of the type described hereinabove; in particular an improved donor element that avoids the need to use pin registration.